1. Field of the Invention
This invention relates generally to electrically conductive power transmission belts, more particularly to static conductive timing belts, and specifically to a belt with conductive film, fabric, and tensile cord.
2. Description of the Prior Art
Drive belts are used for power transmission, motion control, transport applications, as well as for timing applications. Power transmission belts may be in the form of flat belts, toothed belts, V-belts, multi-v ribbed belts or other special profiles. They typically are electrically insulating and/or contain electrically insulating materials. It is sometimes desirable that the belts not retain electrostatic charges, i.e., that the belts have anti-static properties.
Anti-static properties may be defined in various standards and generally include one or more of the following characteristics: surface resistance lower than 108 ohms; volume resistance lower than 109 ohms; and ground bleeder resistance lower than 106 ohms/meter. Standards such as BS PD CLC/TR 50404:2003, DIN EN 13463-1, and IEC 60079-0 provide information on the avoidance of hazards due to static electricity. ASTM D-257 provides a resistance test for various rubber articles. Particularly applicable to timing belts is the standard, ISO 9563:1990(E), which states that the resistance in ohms of new antistatic belts should not exceed 6×105 L/w when measured according to that standard. Herein, a belt that meets the ISO 9563 standard will be called “conductive” whether the belt is new or used. In use, a belt can rapidly lose conductivity from wear or breakdown of the conductive material component.
U.S. Pat. No. 8,192,316 discloses an elastomeric power transmission belt with an anti-static, wear-resistant, covering fabric which includes a nonconductive natural or synthetic polymeric fiber and a conductive fiber. The conductive fiber is a synthetic polymeric fiber with a conductive metallic coating such as silver.
JP1018410A discloses use of a conductive yarn to thwart counterfeiters of toothed belts.
U.S. Pat. No. 6,770,004 discloses a timing belt with an electrically conductive thermoplastic layer on the surface which contacts a pulley. The disclosure claims the conductivity of the belt is maintained over an extended use life, and provides two examples based on a flex tester. The performance in more demanding loaded testing was not reported.
U.S. Pat. No. 4,767,389 discloses a flat, plastic-covered-textile belt with anti-static properties arising from either an electrically conductive filament in the threads of the textile supporting element or an electrically conductive layer between the textile support and plastic covering. The conductive filament may be metal or carbon fiber. The conductive layer may be soot-containing plastic. The plastic covering may be thermoplastic polyurethane. U.S. Pat. No. 7,328,785 teaches a conductive toothed pulley made from a conductive thermoplastic. The thermoplastic may be conductive from the use of conductive microfibers, graphite or carbon black mixed therein.
U.S. Pat. No. 6,228,448 teaches use of an electrically conductive elastomeric surface ply which is preferably doped with a sufficient amount of carbon black or other conductive additives to give the outer ply or entire endless belt a surface resistivity of less than about 1014 ohm/square.
U.S. Pat. No. 5,417,619 teaches a covering canvas impregnated with an anti-static rubber composition based on conductive carbon black. An unwanted side effect of such coatings is a decrease in the abrasion resistance of the fabric resulting in rapid loss of the anti-static effect during use. The resulting abraded particles can be detrimental to nearby electronic or electrical components or systems. U.S. Pat. No. 5,351,530 makes use of such loss of conductivity to indicate the state of wear of a conductive-rubber-coated fabric.
What is needed is a conductive belt which retains conductivity over an extended use life on demanding loaded applications.